This invention relates to thermostats and in particular to electronic digital thermostats which are programmable for automatic operation.
Conventional thermostats employed in the home to control the temperature of an indoor area have generally involved the use of electromechanical devices such as spiral wound bimetallic temperature sensors which expand and contract in response to the ambient temperature of the area. The desired temperature is manually set and the system will control heating and/or cooling to maintain this desired temperature. Both heating and cooling anticipation resistors are often used to interject a known bias into the heating and cooling systems to prevent the system from overshooting the desired temperature.
These conventional systems have several disadvantages. First, only one temperature can be set in, necessitating the user's manually changing the set temperature as desired. This results in wasted energy if the system maintains the same temperature when the space is occupied as when it is unoccupied or during hours when the occupants are asleep. Secondly, the system is often sensitive to small temperature fluctuations, such as might occur when a door is momentarily opened, which causes the system to be activated unnecessarily and more often than needed. Thirdly, the anticipation resistors provide heating and/or cooling bias based on the heating and/or cooling duty cycles rather than the actual rate of change of the ambient temperature during heating and cooling operations. Accordingly, the system may maintain a temperature that is uncomfortably low or high. For example, under mild conditions in the summertime when the cooling system duty cycle is lower than normal, the cooling anticipation resistor exerts a heat bias which exposes the sensor to a temperature that is higher than the actual temperature of the cooled area. This reduces the effective set point and causes the system to run more than is needed and the temperature to be lower than desired.
Recent improvements in thermostat systems have involved the use of two or more thermostat units, each of which is designed to control the temperature during selected time periods. For example, one unit may be used to control temperature during the daytime while a second unit is used to control night temperatures. A clock timer may also be included for timekeeping operations. Such thermostats, for the most part, still use electromechanical relays and switches to control thermostat operations, as exemplified by U.S. Pat. Nos. 3,891,144; 3,197,165 and 4,014,499. An electronically controlled thermostat system for controlling the firing of a fueled heat generator is taught by Haviland, deceased et al (U.S. Pat. No. 4,049,193). This system includes separate night and day thermostats for controlling heating operations during the night and day, respectively, and a digital logic system with a digital clock for timekeeping operations. Four distinct logic circuits are needed to control the heat generator and to switch back and forth between the night and day thermostats.
With the present emphasis on energy conservation, the need for a "smart", inexpensive thermostat which is programmable to maintain desired temperatures during selected time periods so as to minimize energy consumption is widely recognized.